In his May 2007 newsletter Stephen Few, a data visualization guru with an expertise in business data, created the The Graph Design I.Q. Test. "This brief I.Q. test leads you through a series of 10 questions that ask you to choose which of two graphs presents the data more effectively".
I took it myself (and Stephen is probably tracking my answers!) -- it's very cool and quickly teaches a good lesson in good vs. bad graphics and tables. You will be strongly discouraged after it to abuse color, 3D, etc.
If you got hooked, Stephen has a big bag of goodies for those who want to learn about creating good graphs and tables. He wrote a beautiful book called "show me the numbers"
His website's Library also included an abundance of useful articles.